This invention relates to a preset circuit for use with a modulator in an antiskid system, such as an aircraft antiskid system, for example. The general object of this invention is to improve antiskid performance, particularly at the initiation of braking activity.
Brake pressure control on modern, fully modulating antiskid systems is achieved by a combination of a modulator or PBM and a transient control. The modulator attempts to represent the average brake pressure necessary for optimum brake control and is therefore a time integral function of a signal indicative of a braking condition and exhibits relatively slow modulation characteristics. The transient control is a fast pressure control system that corrects instantaneous brake pressure to avoid over modulation of the modulator.
Prior to initial brake application, the modulator has no information regarding runway conditions, and it therefore allows full metered pressure to be applied to the brake. The resulting deep initial skids are used to adjust the modulator to the required average level. Frequently, as many as five deep skids are produced while the modulator is learning and adjusting to the required average brake pressure level. These initial deep skids reduce braking efficiency.
It has been recognized for some time that various types of antiskid systems which utilize modulators may benefit from initialization of the modulator after initial skid activity. Ruof U.S. Pat. No. 3,582,151 discloses one early such system for use with an antiskid system that provides for modulated reapplication of brake pressure. Amberg U.S. Pat. No. 4,180,223 discloses a second example of an antiskid system which provides an initial skid circuit to modify a brake control signal to reduce the tendency of the system to induce initial skids upon initial application of brake pressure. Cook U.S. Pat. No. 4,338,667, which is assigned to the assignee of the present invention, discloses a digital system which initializes the modulator or PBM after initial braking activity.
The Cook system described above functions particularly effectively in a digital system. This is in part due to the fact that the modulator of a digital antiskid system can rapidly be initialized when desired, after initial braking activity. However, in an analog antiskid system the initialization of a modulator typically takes an appreciable period of time. Thus, braking efficiency can be lost in an analog anti-skid system if the modulator is not initialized until after skid activity has begun. Both the Amberg and Ruof systems referenced above suffer from the same disadvantage in that they do not begin to initialize the modulator until after skid activity has commenced.